Updated 6 p.m. | Missouri Senate candidate Rep. Todd Akin apologized for his controversial remarks about "legitimate rape" today, as top Republicans ramped up calls for him to step aside as the GOP nominee.

In the clearest sign yet that pressure is being exerted on Akin to leave the race, National Republican Senatorial Committee Chairman John Cornyn (Texas) issued a statement saying the Congressman has 24 hours to make a decision on what's "best for him, his family, the Republican Party, and the values that he cares about" and to "carefully consider" his options. A GOP source also confirmed that the NRSC is prepared to pull its $5 million ad reservation if Akin does not drop out of the contest.

Cornyn's statement came within hours of several prominent Republicans, including Sens. Scott Brown of Massachusetts and Ron Johnson of Wisconsin, calling for Akin to remove himself from the race for the good of the party. Johnson was a strong backer of John Brunner in Missouri's GOP Senate primary earlier this month.

Though the campaign of GOP presumptive presidential nominee Mitt Romney has condemned Akin's remarks, Romney has not asked Akin to step down, according to both a campaign official and Akin in his interview.

The NRSC chairman's statement also came minutes after Akin's radio interview with Mike Huckabee ended. It was Akin's first public remarks since a media firestorm erupted over his comment that "legitimate rape" rarely leads to pregnancy.

"I've really made a couple of serious mistakes here that were just wrong," Akin told Huckabee. "I care deeply for the victims of people who have been raped ... rape is never legitimate. It is an evil act committed by violent predators. I used the wrong words in the wrong way. What I said was ill-conceived."

"I'd like to apologize to those that I've hurt," Akin continued, in reference to his assertion to a St. Louis television reporter that the "female body has ways to try to shut" down pregnancy in the case of rape.

To make matters worse for Akin's flailing campaign, a high-profile super PAC also announced today it has begun withdrawing from from the Show-Me State. Nate Hodson, a spokesman for American Crossroads and Crossroads GPS, confirmed a Politico report that the group has canceled a new round of ads it planned to begin running this week.

In the interview with Huckabee, Akin did not sound like a man ready to stand down, saying no one has asked him to discontinue his race and expressing confidence in his ability to beat his Democratic opponent, Sen. Claire McCaskill.

"No one has called me and said, 'Todd i think you should drop out.' No one has said that. I gather people are saying that in the media, but no one has called me and said that," Akin told Huckabee when asked if his remarks were "fatal" for his Senate bid. "By the grace of god, we're going to win this race."

Romney is, of course, one of the most politically important voices in a growing chorus against the Missouri Representative, but other influential Republicans have piled on as well. Sean Hannity told Akin while interviewing him that he believed his presence in the race could cost the GOP Missouri's Senate seat, the Senate majority and Romney's chance at Missouri's 10 electoral votes.

Though Akin is showing no signs publicly of backing down, Republican sources have already started circulating names of candidates the Missouri GOP central committee could choose to replace him. Among the names being floated are John Brunner and Sarah Steeleman, both of whom lost to Akin in this month's primary; former Sen. Jim Talent, who lost to McCaskill in 2006; and state Auditor Tom Schweich.

Akin has until Tuesday at 5 p.m. local time to withdraw from consideration without any sort of penalty. He can withdraw as late as Sept. 25 with a court order, but he would have to pay any additional cost incurred by the state to reprint ballots. If he does choose to drop his bid, the state party would have 28 days from his withdrawal to name a replacement. With top leaders putting pressure on Akin to step down and Akin digging in, sources on the ground in Missouri are playing their cards close to the vest, unsure of the party's next move.